An event is something that only occurs once, or only at specific times. Some common examples:
1. The very first time the player enters a screen, a Cut Scene plays. From then on, the cut scene will not trigger, even if the player returns to the room.
2. The player does something on one screen that has an effect on another screen.
Because Cut Scenes usually require moving the player to another screen, events boil down to being able to trigger secrets on one screen from the other. Usually, a hidden stair or pit warp on the first screen is covered with a walkable combo after secrets trigger, preventing the player from encountering the Cut Scene again.
Previously, the only way to accomplish this Remote Screen Secrets Triggering was to use Boss flags. In recent builds, you can use Screen Carryovers or Scripts to accomplish a similar effect.